Solder masks (SM) and photoimagable solder masks (PISM) are a permanent coating for a printed circuit board (PCB) that do not cover certain parts of the circuitry on the board such as the contact points (pads). Two component UV curing acrylates and epoxy-acrylate PSMs are traditionally employed by at least two subsequent steps (i) covering the PCB totally; and (ii) applying various photolithographic techniques to revealing underlined structures on the board while masking other, so that the solder may be applied to the exposed structures. This intricate process has many drawbacks such as requirement for many process steps before final curing, exposure of the board to aggressive developers, contamination throughout the PCB's holes, frequent damage to embedded components, obligation to utilize pollutant solutions that need expensive de-toxification and capital consuming equipment. Moreover, the long-term properties of the cured film are scarified for developability, which calls for water or solvent solubility.
Those drawbacks and other may be prevented by digital application of the solder mask specifically to the targeted portions of the PCB. The most efficient method to do that is by ink-jet technology. Nevertheless, such machinery is by any means limited to liquid ink which is obligatorily characterized by a surface tension lower than 50 dyn/cm at application temperature; viscosity of less than 80 Cp at application temperature; incomparable stability which avoids sedimentation and segregation during storage and jetting; one pack system, wherein the viscosity changes less than 10% within long periods of time, say at least 1 month; very fast development of viscosity after deposition of drop, to keep highest resolution; contains minimal content of volatile matter, usually less than 25% etc. A commercial SM is further characterized by a suitable physical, chemical and mechanical properties to withstand presence of solder, fluxes, organic and inorganic cleaners without any significant deterioration while maintaining its coverage over the portions of the board wherein solder or conductor or glass-epoxy substrate is to be masked, such as required at the IPC SM840-B specifications. The covered board should have at least the same flammability resistance as the original board, according to UL94 V0 specification. Most SMs contains halogenated fire retardants to meet this requirement. Due to environmental regulation it is highly recommended to meet the UL94 without having halogens in the SM.
The chemical and physical resistance of commercially available SMs is correlated to high Tg values, which reflects the high density of the cross-linking and the rigididity of the molecular backbone due to high content of aromatic or heterocyclic structures. The mask should remain flexible enough to withstand mechanical stresses without cracking or peeling and thus to outlast in ever changing environments. In order to meet this difficult trade-off, high molecular weight resins are required, usually having viscosity at ambient temperature, between 5,000 to 100,000 Cp. The chemical resistance of industrial solder masks is mostly achieved by epoxy resins and their curatives comprising respectively high content of aromatic, heterocyclic or cyclo-aliphatic ingredients. For lower end boards, acrylic-based solder mask are applicable, but they are inferior in all respects relatively to the commercially available epoxy-based resins.
Epoxy-based SMs are cost-effective matrices known of their excellent chemical and thermal properties; improved adhesion to metals, ceramics and plastics; ease of application; low toxicity; and comprised a wide spectrum of resins, diluents, modifiers and curing agents. The most popular epoxy resins for high performance applications are diglycidyl ethers of bisphenol A (hereinafter ‘DGEBA’), epoxy phenol novolacs (hereinafter ‘EPN’), epoxy cresol novolacs (hereinafter ‘ECN’), diglycidyl ethers of bisphenol F (hereinafter ‘DGEBF’), commercially available bisphenol A based novolacs products or any mixture thereof.
Epoxy resins are cured by various compositions, such as amines, acids and anhydrides, mercaptans, sulfide and Lewis acids and inorganic salts. Most of the aforementioned curing agents react with the resin at ambient temperature, thus applicable only in two-pack systems, and have a significantly limited pot life.
Latent curatives are practically inactive hardeners until triggered by effective irradiation or heat means. One potential latent system is based on UV-initiated cationic initiators that supply the required Lewis acid to an epoxy system. Hence, U.S. Pat. No. 6,319,652 to Akutsu et al. discloses a energy beam curable epoxy resin composition, essential including cationic polymerizing organic substance; sensitive initiator; organic substance, radically polymerizing organic compound, and radical polymerization initiator. Similarly, U.S. Pat. No. 6,210,862 to Day et al. teaches a very complicated non-brominated epoxy composition for use as solder mask consisting of two systems: (a) epoxy resin having a polyol resin which is a condensation product of epichlorohydrin and bisphenol A; epoxidized multifunctional bisphenol A formaldehyde novolak resin and a solid epoxidized glycidyl ether of bisphenol A; and (b) a photoinitiator capable of initiating polymerization upon exposure to actinic radiation. Unfortunately, these epoxy-base solder masks are expensive, toxic, very sensitive to humidity and contaminations, and can utilize merely narrow spectrum of raw materials, thus limited for practical mass production applications. They have also limit of penetration of light and always requires extra heat post curing for cure completion and thermal relaxation.
Another family of latent curative is based on fine solid curative matrices that are only partially soluble in the epoxy resins at ambient temperature, however, after being melted in the polymer their solubility is increasing and the polymerization is initiate thereon. These curative compounds are usually selected from modified amines, such as the commercially available AJICURE product by Ajinomoto and ANCAMINE 2441 and 2442 by Air Products; imidazoles such as the CUREZOL commercially available by Air Products; dicyandiamide (DICY) such as the AMICURE CG-1400 commercially available by Air Products; urea derivatives; inorganic compounds such as BF3 and BCl3 salts like the LeeCure commercially available catalyst by Leepoxy plastics Inc. In the presence of solvents or low molecular weight co-monomers, which is especially crucial for ink-jet applications, modified amines, imidazoles, BCl3 and BF3 and urea derivatives has some reactivity at ambient temperature, causing the viscosity to increase. Thus those curative compositions are less applicative in mass industrial utilizations but may be used in small-volume applications, where pot life of less than one week is allowed.
DICY has lowest solubility and once having size reduced to less than 2 microns, it becomes very effective latent curing agent for ink jettable epoxy, even when the ink contains solvents and monomers, such as acrylic and methacrylic compositions. DICY cures epoxy to very high Tg values and much alike aromatic amines and anhydride curing agents, it is characterized by excellent chemical and physical properties suitable for solder masks. DICY is a commercially available pulverized powder. The finest powders available are having an average particle size of are of 6 microns and thus excluded from ink jet inks. In order to be applied by ink-jet, DICY should be manipulated to maximal particle size of less than 2 micron, and more preferably less than 700 nanometers, wherein its particles are stable in a manner that their aggregation or agglomeration is avoided.